Different components of a microelectronic component assembly can be attached and electrically coupled to each other in a variety of ways. For example, flip chips typically include a series of bond pads, each of which carries a separate solder ball or “bump.” This “bumped” chip may then be positioned on a substrate with the solder balls contacting an array of electrical contacts carried on a surface of the substrate. By heating the solder, the flip chip can be mechanically joined and electrically coupled to the substrate.
FIG. 1 schematically outlines a conventional process for “bumping” a flip chip and attaching the flip chip to a substrate. This process 10 starts with a chip bearing a plurality of bond pads in step 12. Solder is deposited on the bond pads in step 14. This may be accomplished, for example, by depositing a solder paste through a solder stencil or by attaching pre-formed solder spheres on the bond pads. Solder pastes commonly include a soldering flux which reacts with metal oxides in the solder paste, which can promote melting of the solder metal and wetting of the bond pads with the solder when the solder is heated above a reflow temperature. If solder spheres are used, the solder spheres are typically contacted with a tacky flux composition which helps hold the solder spheres in place as they are heated to a temperature to which they will flow. This heating process, commonly referred to as “reflow,” will metallurgically attach the solder to the bond pads, creating solder balls or bumps on the chip (step 16).
The solder bumps will tend to oxidize. Accordingly, a flux is usually necessary to clean the metal oxides from the solder to attach the chip to the substrate. In step 20, the solder balls are dipped in a flux. This is commonly accomplished using a rotating drum with a supply of an organic flux and a doctor knife to control the thickness of the flux. Thereafter, the chip may be placed on the substrate in step 22. The solder balls on the chip may be reflowed to attach the component to the substrate (step 24), with the flux deposited on the solder balls in step 20 promoting flow of the solder.
Older fluxes commonly left a residue that was unsightly and/or interfered with further processing of the microelectronic assembly or degraded its performance. Accordingly, such flux residues may be cleaned in step 26. Increasingly, so-called “no-clean” fluxes are being employed. Such fluxes typically leave virtually no reside at all or leave a residue which is unlikely to interfere with further processing or use of the microelectronic assembly.
When the flip chip is coupled to the substrate, a gap is commonly left between the flip chip and the opposed surface of the substrate. To further enhance the mechanical bond between the chip and the substrate, this gap may be filled in step 28 with an underfill material, typically an organic resin. This underfill material may also protect the solder joints from chemical attack by moisture or other agents. At the end of the process 10, optionally including the cleaning and underfill steps 26 and 28, a microelectronic component assembly is produced (step 40).